Multifrequency studies of bright radio supernova
remnants 1: 3C 391

Abstract

We report radio observations of the bright, compact supernova remnant 3C 391
using the Very Large Array
of the National Radio Astronomy Observatory (NRAO) at 330, 1468, and 4848 MHz.
We present and
discuss high-resolution images of total intensity, polarization, and spectral
index. The large-scale
morphology consists of a bright partial shell with a considerably larger plateau
of fainter emission
extending past the open end of the shell, suggesting that the remnant is
directly interacting with a dense
region of the interstellar medium, possibly a molecular cloud. The partial shell
may result from gradients
in the external magnetic field that would be expected in the presence of strong
density gradients.
Small-scale extensions beyond the shell edge can be interpreted as due to
relativistic electrons diffusing
upstream of the shock along external magnetic field lines with a mean free path
about an order of
magnitude smaller than characteristic of the mean interstellar medium for
particles of energy a few GeV. If
this interpretation is correct, shock-accelerated electrons are dominantly
produced where the shock normal
is perpendicular to the upstream magnetic field. We find no polarization at 330
or 1468 MHz, with 3 sigma
upper limits of 6 mJy/beam (10 sec beam) at 330 MHz and 0.3 mJy/beam (6 sec
beam) at 1468 MHz. We do
obseve polarized flux at 4848 MHz, but a mean polarized fraction of 0.77% +/-
0.06 %, far lower than
typical for bright supernova remnants. Tangled or disordered magnetic fields in
the emitting region of the
radio shell may be responsible for depolarizing the radio synchrotron radiation,
but some internal Faraday
depolarization may also occur. We crudely estimate the foreground Faraday
rotation to be about -- 500
rad/sq m, consistent with previous estimates. Spectral index images created from
the total intensity images
show no variation beyond delta(alpha) = 0.1. We do see variations at lower
levels which are formally
marginally significant but which are not consistent among the three pairs of
frequencies. Small errors in
the total flux density at each frequency or in the deconvolutions are probably
responsible.